1. Field of the Invention
This invention relates to a method of producing a thermistor element, formed mainly of a metal oxide sintered body, and a production apparatus for producing raw materials for such a thermistor element. The thermistor element can be appropriately used for a thermistor element of a temperature sensor, for an automobile exhaust gas, etc, capable of detecting a temperature from room temperature to a high temperature in the range of 1,000xc2x0 C. or above.
2. Description of the Related Art
Thermistor elements of this kind, and formed mainly of a metal oxide sintered body, have been used in the past for temperature sensors for measuring temperatures from a medium temperature range to a high temperature range of 400 to 1,300xc2x0 C. such as an automobile exhaust gas temperature, a gas flame temperature of gas fed water heaters, a temperature of a heating furnace, and so forth.
Metal oxide sintered bodies made of a perovskite type material, a corundum type material, etc, have been mainly used for the thermistor elements of this kind. A thermistor element using the perovskite type material, for example, is described in Japanese Unexamined Patent Publication (Kokai) No. 7-201528.
To produce a thermistor element that can be used in a broad temperature range, the thermistor element in this reference is obtained by a so-called xe2x80x9csolid phase methodxe2x80x9d that mixes, pulverizes, granulates and sinters a plurality of oxide materials, e.g. Y, Sr, Cr, Fe and Ti, in a predetermined composition ratio.
In the preparation of the raw materials of the thermistor element in the solid phase method described above, mixing and pulverization of a plurality of oxide raw materials are carried out by use of a medium stirring mill, for example. However, mechanical pulverization using the medium stirring mill is essentially not free from the limit of the pulverization capacity, and the mean particle size of the thermistor raw materials after mixing and pulverization is 0.3 xcexcm, as a limit.
Since the particle size of the pulverized starting materials has a limit when pulverization and mixing of the raw materials are simultaneously carried out, uniformity of the composition is not sufficient to obtain a thermistor element having a higher level of accuracy. Therefore, the resulting thermistor element has large variance of resistance, and this variance invites deterioration of temperature accuracy of the temperature sensors using this thermistor element. Temperature accuracy of temperature sensors using the thermistor element according to the prior art is at most xc2x115xc2x0 C. (from room temperature to 800xc2x0 C.).
In the mixing-pulverization operation by use of the medium stirring mill, components of zirconia balls as a pulverization medium mix as impurities into the thermistor raw materials and result in variance of the resistance or invites deviation of a composition from a target composition.
In the temperature sensors of the automobile exhaust gas, there is a great need for a system for detecting exhaust gas temperatures before and after a catalyst for purifying the exhaust gas of gasoline-engine cars to detect deterioration of the catalyst, and for a system for detecting the exhaust temperatures before and after the catalyst to control the temperature of the catalyst for controlling the exhaust gas, particularly a NOx gas, of diesel engines.
However, the temperature accuracy of the temperature sensors using the thermistor element according to the prior art cannot establish this system, and expensive thermocouples or platinum resistors have been used for the temperature sensors. In other words, no temperature sensors are available, to this date, that have temperature accuracy adaptable to the system described above.
In view of the problems described above, the present invention contemplates to reduce variance of the resistance value of the thermistor element when producing the thermistor element formed mainly of the metal oxide sintered boy, and to make further uniform the composition of the thermistor raw materials to obtain a higher level of temperature accuracy.
(I) To begin with, a solution means for obtaining excellent temperature accuracy by forming micro-particles of a thermistor raw material and making uniform the composition will be explained.
To accomplish the object, a first aspect of the invention provides a method of producing a thermistor element consisting of a metal oxide sintered body as a principal component thereof, comprising the steps of mixing a precursor of a metal oxide in a liquid phase and preparing a precursor solution; spraying the precursor solution and obtaining droplet particles; heat-treating the droplet particles and obtaining thermistor raw material powder; and molding and sintering the thermistor raw material powder into a predetermined shape, and obtaining the metal oxide sintered body.
According to this method, mixing of the raw materials can be conducted under the state of the precursor solution. In other words, the composition for obtaining the final metal oxide sintered body can be uniformly regulated in the liquid phase state in which the particles are finer than in the solid phase method according to the prior art. Consequently, the composition of the resulting thermistor raw material powder can be made move uniform. This method is free from mixing of a pulverization medium as an impurity that has been observed in the solid phase method.
The metal oxide sintered body obtained by molding and sintering this raw material powder, that is, the thermistor element, has reduced variance of the resistance value and can provide a higher temperature accuracy than the prior art.
Here, the precursor solution preferably contains at least one kind of metal ion complex.
Water or an organic solvent, or a mixed solution of water and the organic solvent, can be used as the solvent of the precursor solution.
According to a second aspect of the invention, there is provided a method of producing a thermistor element consisting of a metal oxide sintered body as a principal component thereof, comprising the steps of preparing a slurry solution dispersing particles of a metal or a metal oxide; spraying the slurry solution and obtaining droplet particles; heat-treating the droplet particles and obtaining thermistor raw material powder; and molding and sintering the thermistor raw material powder into a predetermined shape, and obtaining the metal oxide sintered body.
According to this method, mixing of the raw materials can be conducted in the form of the slurry solution. In other words, the composition for obtaining the final metal oxide sintered body can be regulated to a uniform composition under the liquid phase state where the particles are finer than in the solid phase method according to the prior art, in the same way as in the first aspect of the invention. Therefore, the composition of the resulting thermistor raw material powder can be made move uniform. This method is free from mixing of the pulverization medium as the impurity as has been the case with the solid phase method.
The metal oxide sintered body formed and sintered by use of this raw material powder, that is, the thermistor element, exhibits reduced variance of the resistance value, and can provide a higher temperature accuracy than the prior art.
To uniformly mix the raw materials, the particle size of the particles of the metal or metal oxide in the slurry solution is preferably 100 nm or below.
The solvent of the slurry solution is preferably water or an organic solvent, or a mixed solution of water and the organic solvent.
The precursor solution or the slurry solution preferably uses a solution to which an inflammable solvent is added and mixed.
In this case, because the inflammable solvent is added and mixed, thermal decomposition and combustion of the droplet particles proceeds rapidly during heat-treatment of the droplet particles sprayed, and the thermistor raw material powder can be obtained with a more uniform composition.
The inflammable solvent is preferably the one selected from the group of methanol, ethanol, isopropyl alcohol, ethylene glycol and acetone.
In the invention, the heat-treating step of the droplet particles uses heating means (5) capable of controlling the temperature in such a fashion that the temperature progressively increases from an inlet of the droplet particles towards an outlet. As a result, the invention can obtain thermistor raw material powder having a sphericalness X, defined by a maximum particle size R max and a minimum particle size R min and expressed by the following equation (1), of at least 80%:
X=(Rmin/Rmax)xc3x97100%xe2x80x83xe2x80x83(1)
The heat-treating step of the droplet particles uses a heating means capable of controlling the temperature in such a fashion that it progressively increases from the inlet of the droplet particles towards the outlet. Therefore, the heat-treating temperature of the droplet particles can be gradually increased.
If the heat-treating temperature of the droplet particles is drastically increased, the droplets rupture and the resulting thermistor raw material powder is likely to become amorphous. When the amorphous thermistor raw material powder is sintered, pores (air entrapment portions inside the sintered body) are likely to develop inside the sintered body.
When the heat-treating temperature of the droplet particles is gradually increased, the raw material powder may become perfect spheres and, when molding and sintering are conducted using the thermistor raw material powder having sphericalness X of at least 80%, the packing property can be improved with the result that pores do not occur. As a thermistor element having a high density and uniform sintered particles can thus be obtained, variance of the resistance value can be further reduced and a high-performance thermistor element can be provided.
The particle size of the droplet particles is preferably not greater than 100 xcexcm. When the particle size of the droplet particles is 100 xcexcm or below, the composition can be made more uniform.
The metal oxide sintered body is a mixed sintered body (M1M2)O3xc2x7AOx of a compound oxide expressed by (M1M2)O3 and a metal oxide expressed by AOx, M1 in the compound oxide (M1M2)O3 is at least one kind of elements selected from the Group 2A and the Group 3A of the Periodic Table with the exception of La, M2 is at least one kind of elements selected from the Groups 3B, 4A, 5A, 6A, 7A and 8 of the Periodic Table, and the metal oxide AOx is a metal oxide having a melting point of 1,400xc2x0 C. or above and a resistance value at least 1,000 xcexa9 at 1,000xc2x0 C. as a single substance of AOx in the form of the thermistor element.
To produce a temperature sensor to be used over a broad temperature range, it is preferred to use a mixed sintered body of a compound oxide (M1M2)O3 of a perovskite structure having relatively low resistance characteristics in a temperature range of room temperature to 1,000xc2x0 C. and a metal oxide AOx having a high resistance value and a high melting point.
When the metal oxide AOx having a melting point of 1,400xc2x0 C. or above and a resistance value of at least 1,000 xcexa9 at 1,000xc2x0 C., as the AOx single substance in the form of the thermistor element, is used, the resistance value of the mixed sintered body in the high temperature range, its melting point and heat-resistance can be increased. Therefore, high temperature stability of the thermistor element can be improved.
In this way, it is possible to obtain a thermistor element the resistance value of which falls within the range of 100 xcexa9 to 100 Kxcexa9 in the temperature range of room temperature to 1,000xc2x0 C., which exhibits a small resistance value change due to thermal history, which is excellent in stability and which can be used in a broad temperature range.
Here, a molar fraction a of the compound oxide (M1M2)O3 and a molar fraction b of the metal oxide AOx in the mixed sintered body (M1M2)O3.AOx preferably satisfy the relation 0.05xe2x89xa6a less than 1.0, 0 less than bxe2x89xa60.95 and a+b=1.
When these molar fractions a and b have the relation described above, the effect of the thermistor described above (resistance value within predetermined range and resistance stability) can be obtained more reliably. Since the molar fractions can be changed in such a broad range, the resistance value and the resistance temperature coefficient can be variously controlled within a broad range when (M1M2)O3 and AOx are appropriately mixed and sintered.
As to the metal elements in the compound oxide (M1M2)O3, it is preferred, practically, that M1 is at least one kind of elements selected from the group consisting of Mg, Ca, Sr, Ba, Y, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb and Sc, and M2 is at least one kind of elements selected from the group consisting of Al, Ga, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Tc, Re, Fe, Co, Ni, Ru, Rh, Pd, Os, Ir and Pt.
In the metal oxide AOx, the metal element A is preferably at least one kind of elements selected from the group consisting of B, Mg, Al, Si, Ca, Sc, Ti, Cr, Mn, Fe, Ni, Zn, Ga, Ge, Sr, Y, Zr, Nb, Sn, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf and Ta.
The metal oxide AOx is at least one kind of metal oxides selected from the group consisting of B2O3, MgO, Al2O3, SiO2, Sc2O3, TiO2, Cr2O3, MnO, Mn2O3, Fe2O3, Fe3O4, NiO, ZnO, Ga2O3, Y2O3, ZrO2, Nb2O5, SnO2, CeO2, Pr2O3, Nd2O3, Sm2O3, Eu2O, Gd2O3, Tb2O3, Dy2O3, Ho2O3, Er2O3, Tm2O3, Yb2O3, Lu2O3, HfO2, Ta2O3, 2MgOxc2x7SiO2, MgSiO3, MgCr2O4, MgAl2O4, CaSiO3, YAlO3, Y3Al3O12, Y2SiO3 and 3Al2Oxc2x72SiO2.
All of these metal oxides have a high resistance value and a high heat resistance and contribute to the improvement of performance of the thermistor element.
In the compound oxide (M1M2)O3, M1 can be Y, M2 can be Cr and Mn, and the metal oxide AOx can be Y2O3.
At this time, the mixed sintered body is Y(CrMn)O3xc2x7Y2O3. This mixed sintered body is appropriately used for the temperature sensors and can exhibit high performance in a broad temperature range.
The mixed sintered body (M1M2)O3xc2x7AOx contains at least one of CaO, CaCO3, SiO2 and CaSiO3 as a sintering aid. Consequently, a thermistor element having a high sintering density can be obtained.
A third aspect of the invention provides an apparatus for producing a raw material of a thermistor element consisting of a metal oxide sintered body as a principal component thereof, comprising spraying means (4) for spraying a precursor solution prepared by mixing a precursor of the metal oxide in a liquid phase and obtaining droplet particles; heating means (5) for heat-treating the droplet particles and obtaining thermistor raw material powder; and recovering means (6) for recovering the thermistor raw material powder; wherein the spraying means, the heating means and the recovering means are interconnected to one another in the indicated order.
Having the construction described above, the production apparatus of the invention can continuously conduct a series of operations such as spraying the precursor solution from the spraying means to form droplet particles, heat-treating the droplet particles by the heating means and recovering the thermistor raw material powder by the recovering means. Therefore, this production apparatus makes it possible to appropriately accomplish the production method of the first aspect of the invention by using the precursor solution, to select the operation time and the scale of the apparatus in accordance with the production quantity and to continuously obtain the raw material powder.
According to a fourth aspect of the invention, there is provided an apparatus for producing a raw material of a thermistor element consisting of a metal oxide sintered body as a principal component thereof, comprising: spraying means (4) for spraying a slurry solution dispersing therein particles of a metal or a metal oxide and obtaining droplet particles; heating means (5) for heat-treating the droplet particles and obtaining thermistor raw material powder; and recovering means (6) for recovering the thermistor raw material powder; wherein the spraying means, the heating means and the recovering means are interconnected to one another in order named.
Owing to the construction described above, the production apparatus of the invention makes it possible to appropriately accomplish the production step of the fourth aspect of the invention by using the slurry solution, to select the operation time and the scale of the apparatus in accordance with the production quantity and to continuously obtain the raw material powder.
A suitable embodiment of the invention includes droplet diameter detecting means (7) for detecting diameters of the droplet particles obtained from the spraying means (4), and wherein the spraying means, the droplet diameter detecting means, the heating means (5) and the recovering means (6) are interconnected to one another in order named.
When the spraying means is regulated on the basis of information of the diameters of the droplet particles obtained from the droplet diameter detecting means, it becomes possible to stabilize the process, to reduce fluctuation among the raw material lots, for example, and to make a contribution to the quality management of the product.
Further, the production apparatus may include arithmetic operation/controlling means (8) for conducting an arithmetic operation and an analysis on the basis of droplet particle data of the droplet diameter measuring means (7), and controlling a spraying condition of the spraying means (4). Therefore, the production apparatus can more reliably execute automatic control, can further stabilize the process and can contribute to quality management of the product.
The spraying means (4) for obtaining the droplet particles is appropriately a two-fluid nozzle, an injection nozzle or a ultrasonic atomizer.
When the atomizing means (4) is the two-fluid nozzle, a gas selected from air, nitrogen and oxygen can be used as a carrier gas for the two-fluid nozzle.
The spraying means (4) is preferably the one that can introduce the flow of the droplet particles under a rotating state into the heating means (5). As the droplet particles move while rotating inside the heating means, the traveling distance of the droplet particles inside the heating means can be advantageously elongated.
An internal pressure of the tank constituted by means from the spraying means (4) to the recovering means (6) interconnected to one another can be kept at a negative pressure. As the internal pressure of the tank is kept at the negative pressure, a smooth flow of the droplet particles can be created. Consequently, a thermistor raw material powder (synthetic raw material) having a more stabilized composition can be obtained.
When the internal pressure of the tank is not the negative pressure, gas introducing means for introducing the gas into an atomization chamber (42) of the spraying means along the flow of the droplet particles generated by the spraying means (4) is preferably provided.
The flow of the gas introduced from the gas introducing means into the atomization chamber can make smooth the flow of the droplet particles sprayed. Therefore, a thermistor raw material powder (synthetic raw material) having a more stabilized composition can be obtained.
The heating means (5) appropriately comprises a quartz hollow tube (52) having an inlet of the droplet particles and an outlet from which the heat-treated thermistor raw material powder comes out, and an electric furnace (51). The electric furnace can constitute at least one temperature zone that is controlled to a predetermined temperature between the inlet and the outlet of the quartz hollow tube.
When the construction of the temperature zone and its temperature are controlled, the temperature can be set in accordance with thermal behavior of the composition of the starting raw materials. Therefore, thermistor raw material powder having a more uniform composition can be synthesized.
The recovering means (6) may include a cyclone, a filter or an electric precipitator. These recovering means are means suitable for recovering the thermistor raw material powder as the powdery raw material.
The recovering means (6) may include a cyclone on the upstream side and the filter or the electric precipitator on the downstream side.
When the cyclone suitable for recovering large amounts of raw material powder having relatively large particles is disposed on the upstream side and the filter or the electric precipitator suitable for recovering small amount of raw material powder having relatively small particle sizes is disposed on the downstream side, it is possible to constitute means suitable for recovering powdery raw material having smaller particle sizes.
The recovering means (6) is preferably operated while its temperature is controlled to 100 to 200xc2x0 C.
From the aspects of the heat-resistance of the filter material and efficiency of the electric precipitator used for the recovering means, the temperature inside the recovering means is preferably 200xc2x0 C. or below, and is preferably at least 100xc2x0 C. so as not to wet the thermistor raw material powder as the steam occurring in the heating means dews in the recovering means.
The invention provides a temperature sensor equipped with the thermistor element that is produced by any of the production methods described above.
The thermistor element produced by the production method described above has reduced variance of the resistance value and has higher temperature accuracy than the prior art level. The temperature sensor sensor using such a thermistor element can detect the temperature over a broad temperature range and can accomplish stable resistance value characteristics and a high-performance temperature sensor because variance of the resistance is small.
Incidentally, a number in parentheses for each means represents an example of correspondence relation to concrete means described in the later-appearing embodiments.
(II) Further, solution means capable of improving temperature accuracy by eliminating the pores of a molding obtained by molding the ceramic raw material powder will be explained.
In other words, the present inventors have conducted intensive studies of the production method of the ceramic element by the solid phase method of the prior art to solve the problems described above, and have discovered that resistance variance can be reduced and temperature accuracy can be improved when pores of a molding (air entrapment portions in a molding) are eliminated.
The solid phase method includes the steps of pulverizing and mixing metal oxide raw materials by use of a medium stirring mill to obtain ceramic raw material powder, mixing a binder for granulating the ceramic raw material with the raw material, granulating the mixture, molding the resulting granulated powder, and sintering the resulting molding.
In the production method by the solid phase method of the prior art, however, mixing and pulverization of the raw materials are simultaneously conducted as described above. In addition, since there is the limit to the particle size of the raw materials so pulverized, the composition of the ceramic element does not become sufficiently uniform. When the components of the pulverization medium mix as impurities into the ceramic low materials, the composition deviates from a target composition of the ceramic element.
Then, the pores occur in the molding obtained by molding, or such pores result in pores in the ceramic element (air entrapment portions in the sintered body constituting the ceramic element) obtained by sintering a molding having a low molding specific gravity due to the existence of the pores.
For this reason, the ceramic element produced by the solid phase method according to the prior art has a low relative specific gravity that is derived from the sintering specific gravity as the actual measurement value and a theoretical specific gravity as a theoretical specific gravity, and the relative specific gravity is generally from 80% to 85%. As a result, the resistance variance closely associated with the internal structure of the ceramic element increases.
Therefore, the present inventors produced the ceramic raw material powder by a liquid phase method. Speaking more concretely, metal oxides or their precursors are dissolved or dispersed and mixed, and droplet particles obtained from the solution are heat-treated to obtain a ceramic raw material powder.
According to this method, mixing of the raw materials can be conducted in the solution form. In other words, the composition for obtaining the final metal oxide sintered body can be uniformly regulated in the liquid phase state where the particles are smaller than in the solid phase method according to the prior art, and the composition of the resulting ceramic raw material powder can be made more uniform. This method is free from mixing of the pulverization medium as the impurity that has been observed in the solid phase method.
However, the following problem occurs when the ceramic raw material powder is prepared by the liquid phase method. The ceramic raw material powder prepared by the liquid phase method directed to attain uniformity of the composition consists of fine particles having a mean particle size of 30 to 50 nm (nano-meters).
Granulated powder suitable for molding by use of a metal mold is prepared by adding a binder, etc, to this ceramic raw powder of the fine particles. Because the particles are fine particles, however, it is difficult to uniformly spread the binder, etc, to be added for granulation, among the particles of the ceramic raw material powder.
As a result, the portions where the binder does not uniformly enter the gaps among the particles form granulated powder in which the ceramic raw material powder is not tightly bonded and pores eventually develop in the molding obtained by metal molding.
In other words, the liquid phase method can solve the problem, of the solid phase method, that the composition of the ceramic raw material powder is not uniform. However, when the liquid phase method is used, a new problem develops in that permeability of the binder mixed with the raw material powder is not sufficient and eventually, the pores occur in the molding or the sintered body (ceramic element) after sintering.
As a result of the analysis of the cases, the present inventors have found that when the mean particle size of the ceramic raw material powder is controlled, the occurrence of the pores in the molding can be eliminated and the relative specific gravity of the ceramic element obtained after sintering can be raised to 90% or more. In this way, the problem described above can be eliminated. The invention is completed on the basis of the observation acquired from the investigation result given above.
A fifth aspect of the invention provides a method of producing a ceramic element formed of a sintered body obtained by sintering a ceramic raw material made of a metal oxide, wherein raw material powder produced by a liquid phase method and having a mean particle size of 0.1 to 1.0 xcexcm is used as the ceramic raw material, and the ceramic raw material is granulated, molded and sintered so that the sintered by has a relative specific gravity X, defined by a sintering specific gravity and a theoretical specific gravity, of at least 90% as expressed by the following equation (2):
xe2x80x83relative specific gravity X=(sintering specific gravity/theoretical specific gravity)xc3x97100%xe2x80x83xe2x80x83(2)
By using the liquid phase method, the invention can make the composition of the ceramic raw material further uniform.
Studies conducted by the present inventors have experimentally revealed that when the mean particle size of the ceramic raw material powder produced by the liquid phase method is within the range of 0.1 to 1.0 xcexcm, the binder uniformly permeates among the particles of the raw material powder when the granulated powder is formed by mixing the binder with the raw material powder.
Therefore, the ceramic raw material powder is bonded mutually and tightly to form the granulated powder. In the molding obtained by molding such granulated powder, the occurrence of the pores can be suppressed, and a ceramic element formed of the sintered body having a relative specific gravity X of at least 90% can be obtained.
As described above, the invention can make the composition of the ceramic raw materials more uniform than in the prior art method, and can reduce variance of the resistance value of the ceramic element by reducing the pores and improving the relative specific gravity X.
A sixth aspect of the invention provides a method of producing a ceramic element formed of a sintered body obtained by sintering a ceramic raw material made of a metal oxide, comprising the steps of mixing a precursor of the metal oxide in a liquid phase and preparing a precursor solution; spraying the precursor solution and obtaining droplet particles; conducting a first heat-treatment step of heat-treating the droplet particles and obtaining raw material powder of the ceramic element; conducting a second heat-treatment step of heat-treating the raw material powder obtained by the first heat-treatment step at a temperature higher than that of the first heat-treatment step, and changing a mean particle size of the raw material powder to 0.1 to 1.0 xcexcm; and granulating, molding and sintering the raw material obtained by the second heat-treatment step.
According to this method, mixing of the raw materials can be made in the state of the precursor solution, that is, by the liquid phase method, before the first heat-treatment step. Therefore, the composition of the ceramic raw material can be made move uniform.
The second heat-treatment step allows the fine particles of the raw material powder obtained by the liquid phase method to grow to a mean particle size of 0.1 to 1.0 xcexcm. Therefore, when the mixture of this raw material powder and the binder are used to form the granulated powder in the same way as in the fifth aspect of the invention, the binder uniformly permeates the particles, and the ceramic raw material powder is converted to a granulated powder in which the particles are tightly bonded to one another. As a result, the occurrence of the pores in the molding can be suppressed.
Therefore, the invention can make the composition of the ceramic raw materials much more uniform than can the prior art method. Because the invention reduces the pores and improves the relative specific gravity X(Xxe2x89xa790%), it can reduce variance of the resistance value of the ceramic element.
A seventh aspect of the invention provides a method of producing a ceramic element formed of a sintered body obtained by sintering a ceramic raw material made of a metal oxide, comprising the steps of preparing a slurry solution dispersing therein particles of a metal or a metal oxide having a mean particle size of 1.0 xcexcm or below; spraying the slurry solution and obtaining droplet particles; conducting a first-heat-treatment step of heat-treating the droplet particles and obtaining raw material powder of the ceramic element; conducting a second heat-treatment step of heat-treating the raw material powder obtained by the first heat-treatment step at a temperature higher than that of the first heat-treatment step, and changing a mean particle size of the raw material powder to 0.1 to 1.0 xcexcm; and granulating, molding and sintering the raw material obtained by the second heat-treatment step.
In the first heat-treatment step, the mixture of the raw materials can be regulated to a uniform composition for obtaining the final sintered body under the liquid state where the particles are much smaller than in the solid phase method of the prior art, in the same way as in the sixth aspect of the invention. Therefore, the resulting composition of the ceramic raw material powder can be made move uniform.
The second heat-treatment step allows the particles of the fine raw material powder obtained by the liquid phase method to grow move and the mean particle size can be changed to 0.1 to 1.0 xcexcm. Consequently, the binder uniformly permeates the particles in the same way as in the fifth aspect of the invention, and the granulated powder in which the raw material powder is bonded mutually tightly can be prepared. Eventually, the occurrence of the pores can be suppressed in the molding.
Therefore, this invention can make the ceramic raw material composition much more uniform than the prior art method, can reduce the pores and can improve the relative specific gravity X(Xxe2x89xa790%). As a result, the invention can reduce variance of the resistance value of the ceramic element.
An eighth aspect of the invention provides a method of producing a ceramic element formed of a sintered body obtained by sintering a ceramic raw material made of a metal oxide, comprising the steps of mixing a precursor of the metal oxide in a liquid phase and preparing a precursor solution; preparing a dispersion solution by dispersing particles of a metal or a metal oxide having a mean particle size of not greater than 1.0 xcexcm in the precursor solution; spraying the dispersion solution and obtaining droplet particles; conducting a first heat-treatment step of heat-treating the droplet particles and obtaining raw material powder of the ceramic element; conducting a second heat-treatment step of heat-treating the raw material powder obtained by the first heat-treatment step at a temperature higher than that of the first heat-treatment step, and changing a mean particle size of the raw material powder to 0.1 to 1.0 xcexcm; and granulating, molding and sintering the raw material obtained by the second heat-treatment step.
According to this method, the mixing of the raw materials can be uniformly regulated to the composition for obtaining the final sintered body under the liquid phase state, in which the particles are smaller than in the solid phase method of the prior art, before the first heat-treatment step in the same way as in the sixth aspect of the invention. Therefore, the composition of the resulting ceramic raw material powder can be made move uniform.
The second heat-treatment step allows the particles of the fine raw material powder obtained by the liquid phase method to grow move, and the mean particle can be changed to 0.1 to 1.0 xcexcm. Therefore, the binder uniformly permeates among the particles in the same way as in the fifth aspect of the invention, and the granulated powder in which the raw material powder is bonded mutually tightly can be prepared. Eventually, the occurrence of pores can be suppressed in the molding.
Therefore, this invention can make the ceramic raw material composition much more uniform than the prior art method, can reduce the pores and can improve the relative specific gravity X(Xxe2x89xa790%). As a result, the invention can reduce variance of the resistance value of the ceramic element.
In the production method described in any of the fifth to eighth aspects of the invention, the moisture ratio of the granulated powder obtained after granulation of the raw material power can be appropriately set to 3% or below.
The mixture of the raw material powder and the binder is granulated, and the resulting granulated powder is molded by use of a metal mold. In this case, the granulated powder must smoothly flow into the mold. To conduct molding without forming a bridging inside the mold, the moisture ratio of the granulated powder is preferably 3% or below.
When the moisture ratio of the granulated powder is 3% or below, the bridging of the granulated powder inside the mold can be eliminated. In consequence, a molding free from the pores can be obtained, and the relative specific gravity of at least 90% can be accomplished. Here, the term xe2x80x9cmoisture ratioxe2x80x9d represents the proportion of the moisture (percentage) contained in the granulated powder, and can be measured by use of a known moisture meter.
In the production method described in any of the fifth to eighth aspects of the invention, a bulk specific gravity of the molding obtained after granulation and molding of the raw material powder can be at least 50%.
When the bulk specific gravity of the molding formed by molding the granulated powder obtained by granulation of the raw material powder is set to at least 50%, the occurrence of the pores inside the ceramic element obtained after sintering this molding can be prevented, and a ceramic element satisfying the relative specific gravity of at least 90% can be easily obtained.
When the raw material powder having a mean particle size of 0.1 to 1.0 xcexcm is used to prepare the granulated slurry in the production method described in any of the fifth to eighth aspects of the invention, the raw material powder is converted to spheres through the pulverization operation. In this case, the raw material powder can be converted to powder having sphericalness Y, defined by the maximum particles size R max and the minimum particle size R min and expressed by the following equation (1), of at least 80%:
Y=(Rmin/Rmax)xc3x97100(%)xe2x80x83xe2x80x83(1)
The invention relates to the shape of the raw material powder described above.
The granulated slurry prepared from the mixture of the raw material powder and the binder is used to form the granulated powder. When this granulated powder is molded by use of the metal mold, the granulated powder must smoothly flow into the mold. The granulated powder preferably comprises perfect spheres to conduct molding without forming the bridging inside the mold.
Studies made by the present inventors have revealed that sphericalness Y of the raw material powder is preferably 80% or more to obtain the granulated powder of prefect spheres. In this case, the granulated powder becomes more spherical. Therefore, the bridging of the granulated powder inside the mold can be eliminated in the same way as in the eighth aspect of the invention. It is therefore possible to obtain the molding free from the pores and to easily accomplish the relative specific gravity of 90% or more.
The present inventors have furthered their studies concerning the binder to be added to the ceramic raw material powder for granulating the ceramic raw material powder, and have found that the condition of the pores of the molding varies depending on a degree of polymerization and a degree of saponification of the binder.
In other words, the crushing property of the granulated power varies depending on the properties of the binder to be added. When the granulated powder is not easily crushed, the particles of the ceramic raw material powder are not tightly bonded to one another and eventually, pores occur in the molding.
As a result of the analysis of the cause described above, the pores of the molding can be eliminated and the specific gravity of the ceramic element obtained after sintering can be improved to 90% or more.
A ninth aspect of the invention is based on the observation given above, and provides a method of producing a ceramic element formed of a sintered body obtained by mixing a binder for granulating ceramic raw material powder with the ceramic raw material power made of a metal oxide and sintering the mixture, wherein the ceramic powder is prepared by a liquid phase method, the binder is an organic binder having a degree of polymerization of 2,000 or below and a degree of saponification of at least 45%, and the mixture of the ceramic raw material powder and the organic binder is granulated, molded and sintered so that the sintered body has a relative specific gravity X, expressed by the following equation (2), of at least 90%
First, as this invention uses the liquid phase method, it can make the composition of the ceramic raw material powder move uniform.
Studies made by the present inventors have experimentally revealed that when an organic binder having a degree of polymerization of 2,000 or below and a degree of saponification of at least 45% is used as the binder, the binder uniformly permeates into the gaps among the particles of the raw material powder when the mixture of the raw material powder and the binder is molded, irrespective of the mean particle size of the ceramic raw material powder. In other words, it has been found out that when the organic binder is added, fluidity and the collapsing property of the granulated powder can be improved, and a molding free from the pores can be obtained.
Therefore, the granulated power becomes one in which the particles of the ceramic raw material powder are tightly bonded to one another. In the molding obtained by molding such granulated powder, the occurrence of the pores can be suppressed, and a ceramic element comprising the sintered body having a relative specific gravity of at least 90% can be obtained.
Therefore, this invention can make the ceramic raw material composition much more uniform than the prior art method, can reduce the pores and can improve the relative specific gravity X. As a result, the invention can reduce variance of the resistance value of the ceramic element.
At least one member selected from the group consisting of polyvinyl alcohol, polyacetal and polyvinyl acetate alcohol can be appropriately used as the organic binder described above.
Preferably, the ceramic element is ceramic element is a thermistor element formed of a mixed sintered body (M1M2)O3xc2x7AOx of a compound oxide expressed by (M1M2)O3 and a metal oxide expressed by AOx, M1 in the compound oxide (M1M2)O3 is at least one kind of elements selected from the Group 2A and the Group 3A of the Periodic Table with the exception of La, M2 is at least one kind of elements selected from the Groups 3B, 4A, 5A, 6A, 7A and 8 of the Periodic Table, and the metal oxide AOx is a metal oxide having a melting point of 1,400xc2x0 C. or above and a resistance value of at least 1,000 xcexa9 at 1,000xc2x0 C. as a single substance of AOx in the form of said thermistor element.
When the ceramic element is used as a thermistor element for a temperature sensor that is used in a broad temperature range, it is advisable to use a mixed sintered body (M1M2)O3 of a compound oxide of a perovskite structure having relatively low resistance characteristics from room temperature to 1,000xc2x0 C. and a metal oxide AOx having a high resistance value and a high melting point.
When a metal oxide having a melting point of 1,400xc2x0 C. or above and a resistance value of at least 1,000 xcexa9 at 1,000xc2x0 C. as a single substance of AOx in the form of said thermistor element is used, the resistance value of the mixed sintered body in the high temperature range can be elevated, and its melting point and heat resistance can be raised. Therefore, high temperature stability of the thermistor element can be improved.
Accordingly, the invention can provide a thermistor element having a resistance value of 100 xcexa9 to 100 Kxcexa9 in the temperature range of room temperature to 1,000xc2x0 C., exhibiting a small change of the resistance value due to thermal history, excellent in stability and usable in a broad temperature range.
Here, it is preferred that a molar fraction a of the compound oxide (M1M2)O3 and a molar fraction b of the metal oxide AOx in the mixed sintered body (M1M2)O3xc2x7AOx satisfy the relation 0.05xe2x89xa6a less than 1.0, 0 less than bxe2x89xa60.95 and a+b=1.
When these molar fractions a and b satisfy the relation described above, the thermistor element can more reliably accomplish the intended effects (resistance value within a predetermined range and resistance stability). Because the molar fractions can be changed in such a broad range, the resistance value and the resistance temperature coefficient can be variously controlled within a broad range when (M1M2)O3 and AOx are appropriately mixed and sintered.
As to each metal element in the compound oxide (M1M2)O3, it is preferred from the aspect of the practical application that M1 in the compound oxide (M1M2)O3 is at least one kind of elements selected from the group consisting of Mg, Ca, Sr, Ba, Y, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb and Sc, and M2 is at least one kind of elements selected from the group consisting of Al, Ga, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Tc, Re, Fe, Co, Ni, Ru, Rh, Pd, Os, Ir and Pt.
Concrete examples of the metal element A in the metal oxide AOx are at least one kind of elements selected from the group consisting of B, Mg, Al, Si, Ca, Sc, Ti, Cr, Mn, Fe, Ni, Zn, Ga, Ge, Sr, Y, Zr, Nb, Sn, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf and Ta.
The metal oxide AOx is at least one kind of metal oxides selected from the group consisting of B2O3, MgO, Al2O3, SiO2, Sc2O3, TiO2, Cr2O3, MnO, Mn2O3, Fe2O3, Fe3O4, NiO, ZnO, Ga2O3, Y2O3, ZrO2, Nb2O3, SnO2, CeO2, Pr2O3, Nd2O3, Sm2O3, Eu2O, Gd2O3, Tb2O3, Dy2O3, Ho2O3, Er2O3, Tm2O3, Yb2O3, Lu2O3, HfO2, Ta2O5, 2MgOxc2x7SiO2, MgSiO3, MgCr2O4, MgAl2O4, CaSiO3, YAlO3, Y3Al5O12, Y2SiO5 and 3Al2Oxc2x72SiO2.
All these metal oxides exhibit high resistance values and high heat resistance, and contribute to the improvement of performance of the thermistor element.
It is preferred that in the compound oxide (M1M2)O3, M1 is Y, M2 is Cr and Mn and the metal oxide AOx is Y2O3.
At this time, the mixed sintered body is Y(CrMn)O3xc2x7Y2O3. This mixed sintered body is appropriately used for the temperature sensor and can exhibit high performance in a broad temperature range.
The mixed sintered body (M1M2)O3xc2x7AOx contains at least one member selected from CaO, CaCO3, SiO2 and CaSiO3 as a sintering aid. Therefore, a ceramic element as a thermister device having a high sintering density can be obtained.
The invention further provides a temperature sensor having the ceramic element produced by any of the production methods described above as a thermistor element.
The ceramic element produced by the production methods described above reduces variance of the resistance value and has higher temperature accuracy than the prior art level. The temperature sensor using such a ceramic element as the thermistor element can detect the temperature in a broad temperature range and can provide a high-performance temperature sensor because the resistance variance is small.
Incidentally, numbers in parentheses represent a correspondence relation to concrete means described in the later-appearing embodiments.